The present invention pertains to managing the operation of a semiconductor device under varying load conditions that affect the power being dissipated by the semiconductor device.
The power dissipated by a semiconductor device is dissipated as heat, which causes the temperature of the semiconductor device to rise. After prolonged continuous usage, the temperature of an operating semiconductor device, such as a power amplifier, may rise to such an extent that the semiconductor device overheats and fails. In order to prevent such failure, the operation of the semiconductor device is managed by determining the junction temperature of the semiconductor device and cutting back or interrupting the operation of the semiconductor device when the junction temperature reaches a specified maximum attainable junction temperature (TJMAX) for the semiconductor device. The junction temperature is the temperature at the junction of the output transistor of the semiconductor device.
Typically a temperature related to the junction temperature of the semiconductor device is measured by a temperature sensor disposed on the case of the semiconductor device. While such a measurement provides an approximate measurement of the junction temperature, the difference between the temperature of the case at the location of the temperature sensor and the actual junction temperature varies under varying operating conditions of the semiconductor device because such difference is affected by the power dissipation of the semiconductor device.
For semiconductor devices, such as radio frequency (RF) power amplifiers that experience varying load (antenna) conditions, it is desirable to use a semiconductor device that has a specified maximum attainable junction temperature (TJMAX) that guarantees safe operating temperatures for high power-dissipation loads. However, because TJMAX is specified as an absolute maximum by the semiconductor manufacturer, the sensor temperature cutback threshold (TCB) must be overly conservative to account for the worst case power dissipation conditions which vary as the load changes for static thermal management implementations.
Also, since power dissipation is estimated to be the difference between the power supplied to the semiconductor device and the power delivered by the semiconductor device to the load, it is difficult to quantify power dissipation for non-sinusoidal time-varying signals, modulated signals, and loads that cause reflected power at high frequencies.